1. Field of the Invention
The present invention relates generally to an antenna cosite analysis system, and more particularly to an antenna cosite analysis system that is integrated with a tower and antenna layout system so as to enable more efficient design of antenna towers. The invention also relates to a faster antenna cosite analysis system in which intermodulation products that are not capable of causing interference are not processed.
2. Description of the Related Art
Antennas for wireless communication systems are often positioned on towers to improve the operating distance of the systems. Cellular telephone, AM/FM radio, paging services, and mobile telephone are just a few examples of wireless systems that utilize tower-mounted antennas. In order to reduce the cost of maintaining such towers and of operating wireless services, it is preferable for as many wireless systems as possible to use a given tower.
When multiple transmitters, receivers, and/or antennas are located on a tower, several types of interference may occur. Intermodulation occurs when the RF signal from one transmitter at the site leaks into other transmitters or receivers at the site causing intermodulation products to be generated. If the intermodulation products occur in a transmitter, they may escape from the transmitter and cause interference in the receivers at the site.
Another type of interference occurs when noise from the transmitters at the site interferes directly with the receivers at the site. A still further type of interference occurs when the power of the transmitters leaks into the receivers, thereby desensitizing the receivers.
Systems have been developed that predict the extent of interference between transmitters and receivers at a shared radio site. Such systems are described, for example, in the following documents, the contents of which are incorporated herein by reference for all purposes:
1) M. N. Lustgarten, "COSAM (Co-site Analysis Model)," IEEE Electromagnetic Compatibility Symposium Record, Anaheim, Calif., pp. 394-406, July 1970; PA1 2) J. W. Rockway, and S. T. Li, "Design Communication Algorithm (DECAL)," IEEE International Symposium on Electromagnetic Compatibility, Atlanta, Ga., pp. 288-292, June 1978; PA1 3) L. C. Minor, F. M. Koziuk, J. W. Rockway, and S. T. Li, "PECAL: A New Computer Program for the EMC Performance Evaluation of Communication Systems in a Cosite Configuration," IEEE International Symposium on Electromagnetic Compatibility, Atlanta, Ga., pp. 295-301, June 1978; PA1 4) P. Alexander, P. Magis, J. Holtzman, S. Roy, "A methodology for interoperability analysis," IEEE Military Communications Conference (MILCOM 89), Boston, Mass., pp. 905-910, October 1989; PA1 5) J. Low and A. S. Wong, "Systematic approach to cosite analysis and mitigation techniques," Proceedings of the Tactical Communications Conference, vol. 1, pp. 555-567, April 1990; and PA1 6) ComSitePlus User Manual, Douglas Integrated Software, Tallahassee, Fla., 1995. PA1 means for interactively designing an antenna tower for placement of one or more antennas on the tower; PA1 means for interactively designing wireless communication circuits; PA1 means for interactively relating the antennas on the tower to the wireless communication circuits; and PA1 RF analysis means for simulating the wireless communication circuits with their related antennas and for generating reports on the possible interference between the wireless communication circuits. PA1 a) at each transmitter, determining a minimum susceptibility to interference of the receivers; PA1 b) separately considering each transmitter as a victim transmitter paired with each other transmitter, and for each such other transmitter-victim transmitter pair, determining a highest order intermodulation product generated as a result of interference between the victim transmitter and the other transmitter with a power level sufficient to exceed the minimum susceptibility of the one or more receivers at the victim transmitter, and storing the highest order intermodulation product for each other transmitter-victim transmitter pair; and PA1 c) separately considering each transmitter as a victim transmitter, and for each victim transmitter determining the intermodulation products generated as a result of interference between the victim transmitter and all of the at least one other transmitter, each other transmitter having a leakage power to the victim transmitter, the one of the other transmitters having the lowest leakage power to the victim transmitter being the minimum leakage transmitter for that victim transmitter, the minimum leakage transmitter and the victim transmitter together constituting a minimum leakage transmitter-victim transmitter pair, the intermodulation products for each victim transmitter being determined in this step up to the highest order intermodulation product determined in step (b) for the other transmitter-victim transmitter pair that is the same as the minimum leakage transmitter-victim transmitter pair. PA1 for each individual transmitter at each order, determining the power level P.sub.INTatTX for such transmitter as sum of the power leakage L.sub.T2,T1 between such transmitter and the victim transmitter and the conversion loss for the victim transmitter at such order; and PA1 comparing the power level P.sub.INTatTX to the minimum susceptibility at the victim transmitter. PA1 a) determining a minimum susceptibility to interference of the receiver; PA1 b) for each transmitter, determining a highest order intermodulation product generated as a result of interference between the transmitter and the receiver with a power level sufficient to exceed the minimum susceptibility of the receiver, and storing the highest order intermodulation product for each transmitter; and PA1 c) separately considering each transmitter as a victim transmitter, and for each victim transmitter determining the intermodulation products generated in the receiver as a result of interference between the victim transmitter and all of the at least one other transmitter, each other transmitter having a leakage power to the victim transmitter, the one of the other transmitters having the lowest leakage power to the receiver being the minimum leakage transmitter, the intermodulation products being determined in this step up to the highest order intermodulation product of the minimum leakage transmitter determined in step (b). PA1 means for determining a minimum susceptibility to interference of the one or more receivers at each transmitter; PA1 means for separately considering each transmitter as a victim transmitter and for determining and storing a highest order intermodulation product generated as a result of interference between the victim transmitter and each other transmitter individually with a power level sufficient to exceed the minimum susceptibility at the victim transmitter; and PA1 means for separately considering each transmitter as a victim transmitter and for determining the intermodulation products generated as a result of interference between the victim transmitter and all of the at least one other transmitter, each other transmitter having a leakage power to the victim transmitter, the one of the other transmitters with the lowest leakage power to the victim transmitter being the minimum leakage transmitter, the intermodulation products being determined up to the highest order intermodulation product stored for the minimum leakage transmitter and victim transmitter. PA1 means for determining a minimum susceptibility to interference of the receiver; PA1 means for determining and storing, for each transmitter, a highest order intermodulation product generated as a result of interference between the transmitter and the receiver with a power level sufficient to exceed the minimum susceptibility of the receiver; and PA1 means for separately considering each transmitter as a victim transmitter and for determining the intermodulation products generated in the receiver as a result of interference between the victim transmitter and all of the at least one other transmitter, each other transmitter having a leakage power to the victim transmitter, the one of the other transmitters having the lowest leakage power to the receiver being the minimum leakage transmitter, the intermodulation products being determined up to the highest order intermodulation product of the minimum leakage transmitter.
When the interference between the communication circuits on a tower exceeds a desired threshold, the antennas, transmitters and receivers at the site may be relocated or additional filtering elements may be added to the communication circuits in order to reduce the interference to within acceptable limits.
In general, the calculation of direct interference between transmitters and receivers, i.e., non-intermodulation interference, is relatively simple and will not be discussed herein in further detail. The computation of intermodulation interference, however, will be described with respect to a typical antenna site containing .alpha. number of transmitters and .beta. number of receivers, as shown in FIG. 1. In a conventional cosite analysis system, each intermodulation product frequency F.sub.INT and each intermodulation product bandwidth BW.sub.INT is calculated as follows: EQU F.sub.INT =.+-.M.sub.1 .times.F.sub.T1 .+-.M.sub.2 .times.F.sub.T2 + . . . .+-.M.sub..alpha. .times.F.sub.T.alpha. EQU BW.sub.INT =M.sub.1 .times.BW.sub.T1 +M.sub.2 .times.BW.sub.T2 + . . . +M.sub..alpha. .times.BW.sub.T.alpha.
where [M.sub.1,M.sub.2, . . . M.sub..alpha. ] are positive integers.
The order of F.sub.INT is M.sub.1 +M.sub.2 + . . . +M.sub..alpha..
As shown below, each intermodulation product frequency F.sub.INT is then compared to the frequency band of each receiver to determine if the intermodulation product falls within the frequency band of any receiver. ##EQU1##
If an intermodulation product falls within the band of a receiver RX.sub..beta. (the "victim receiver") at frequency F.sub.R.beta., then the power of the intermodulation product is calculated. As illustrated in FIG. 2, for any pair of transmitters, TX.sub.1 and TX.sub.2, the power leakage LT.sub.T2,T1 from TX.sub.2 to TX.sub.1 is calculated as follows: EQU L.sub.T2,T1 =P.sub.T2 -C.sub.T2,T1
where P.sub.T2 is the power of TX.sub.2 and C.sub.T2,T1 is the coupling loss from TX.sub.2 to TX.sub.1 at F.sub.T2.
The power leaking into the target transmitter TX.sub.1 from each other transmitter is calculated in a similar fashion. The transmitter with the lowest leakage power L.sub.MIN to the target transmitter is used to calculate the intermodulation product power level. EQU L.sub.MIN =min(L.sub.T2,T1, . . . ,L.sub.T.alpha.,T1)
Referring to FIG. 3, the intermodulation product power level P.sub.INTatT1 at TX.sub.1 is then calculated as follows: EQU P.sub.INTatT1 =L.sub.MN +ConversionLoss(order)+FL.sub.T1 (atF.sub.INT)
where TX.sub.1 is the victim transmitter, i.e., the transmitter in which the mixing is occurring, L.sub.MIN is the minimum power leakage as calculated above, ConversionLoss is a lookup table unique to the victim transmitter (discussed in detail below), and FL is the output filter unique to victim transmitter. Order is the order of the intermodulation product under consideration.
The intermodulation product level P.sub.INT is then calculated at victim receiver RX.sub..beta. by adding the coupling losses C.sub.T1,R.beta. between the victim transmitter TX.sub.1 and the receiver RX.sub..beta. as follows: EQU P.sub.INTatRX =P.sub.INTatTX +C.sub.T1,R.beta.
where the coupling losses C.sub.T1,R.beta. include all losses between the victim transmitter and the victim receiver.
Referring to FIG. 4, the final step in the analysis is to compare the intermodulation product power level P.sub.INTatRX at the receiver R.sub..beta. to the susceptibility to interference of the receiver S.sub.R.beta.. If P.sub.INTatRX .gtoreq.S.sub.R.beta. then interference occurs.
In prior cosite analysis systems, this process is repeated for each intermodulation product, even for those with power levels below that necessary to cause interference in the most susceptible receiver. Only once the intermodulation product frequencies have been calculated are the power levels calculated for the intermodulation products. Since the actual number of possible intermodulation product frequencies is unlimited, these systems include several provisions to limit computing time to within acceptable limits.
The first provision of those systems is to limit the number of transmitter frequencies mixing together at any given time to a relatively low number, generally two, or at most three. This is because the number of frequency combinations at a complicated site, i.e., a site with many frequencies, increases by approximately the power of the number of frequencies being mixed. For example a site with 100 transmit frequencies will have approximately 100.sup.5 -100.sup.2 .congruent.100.sup.5 more combinations of 5 frequencies mixing than with combinations of two frequencies mixing. This equates to an increase of 100.sup.5 /100.sup.2 =1,000,000 times longer in analysis time.
The second provision of those systems is to compute combinations of frequencies only up to a relatively low order, usually 5.sup.th or 7.sup.th order. The number of combinations for n orders is roughly n(n-1).congruent.n.sup.2. So, for example, a system that calculates combinations of frequencies up to 11 orders will have approximately 11.sup.2 /5.sup.2 .congruent.5 times more combinations that a system that only calculates 5 combinations.
Because the number of intermodulation products greatly increases as higher orders of intermodulation products are considered and as intermodulation products are calculated for more combinations of transmitters, conventional systems significantly limit each of these parameters with concomitant loss of quality of the analysis. Accordingly, it would desirable to have a cosite analysis system in which it is possible to limit the number of intermodulation products that are considered by the system, even while higher orders of intermodulation products are considered and as intermodulation products are calculated for more combinations of transmitters, so as to reduce the amount of time required to analyze a site while improving the quality of the resulting analysis.
Another aspect of cosite analysis relates to the physical placement of antennas and other equipment on the tower. Typically, a civil engineer is responsible for the physical layout of the tower, including locating equipment on the tower and preparing drawings of the tower while the above-described type of analysis of possible RF interference is being conducted. If as a result of either the RF analysis or of the physical constraints of the tower it is necessary to move a piece of equipment on the tower, the tower layout and the RF analysis must both be redone. Thus, because the layout and RF aspects of the layout are not integrated, tower design is less efficient. The RF analysis and the tower layout must also be redone if any equipment is added to or removed from the tower.
Accordingly, it would be desirable to have an antenna cosite analysis system that is integrated with a tower layout system so as to enable more efficient design of antenna towers.